LED (Light Emitting Diode) has advantages of compactness, longer lifespan, superior color performance, lower power consumption, and higher light emitting efficiency. Under the trend of environmental protection, LED has massively replaced the conventional lighting sources. Since an important breakthrough of gallium nitride LED was made in 1993 in Japan, gallium nitride LED has become a hot field of research around the world.
Refer to FIG. 1 for a conventional LED structure. The conventional LED structure 10 comprises a substrate 11, an N-type semiconductor layer 12, a light emitting layer (or called the active layer) 13, a P-type semiconductor layer 14, a P-type current spreading layer 15, a first electrode 16 and a second electrode 17. The N-type semiconductor layer 12 is formed on the substrate 11. The light emitting layer 13 is formed on the N-type semiconductor layer 12. The P-type semiconductor layer 14 is formed on the light emitting layer 13. The P-type current spreading layer 15 is formed on the P-type semiconductor layer 14. The first electrode 16 is formed on the P-type current spreading layer 15. The second electrode 17 is formed on the exposed surface of the N-type semiconductor layer 12. The N-type semiconductor layer 12 may be an N-type gallium nitride (GaN) layer. The P-type semiconductor layer 14 may be a P-type gallium nitride (GaN) layer. The light emitting layer 13 may be an indium gallium nitride (InGaN) layer.
Between the first electrode 16 and the P-type semiconductor layer 14 of a planar or large-area LED, there is a greater sheet resistance likely to generate current crowding. Thus, the P-type current spreading layer 15 is interposed there between to improve current crowding and increase light emitting efficiency.
The conventional LED adopts a P-type current spreading layer 15 made of a nickel-gold or chromium-gold alloy to increase uniformity of current distribution. However, the nickel-gold alloy and the chromium-gold alloy have inferior light permeability. Thus, the thickness thereof must be constrained to hundreds of Å to obtain better light permeability. Nevertheless, a stable film is hard to be attained if the thickness thereof is too thin. Therefore, it is difficult to keep the balance between light permeability and current distribution uniformity.
In recent years, TCO (Transparent Conductive Oxide) films have gradually replaced the abovementioned metal alloy film to function as the P-type current spreading layer 15, whereby the light permeability is improved. Although the TCO film has 90% light permeability and above, the ohmic contact between the TCO film and the P-type semiconductor layer 14 is poor, which is likely to generate current crowding and lower the total light emitting efficiency. There are many prior arts for improving poor ohmic contact. For an example, R.O.C. patent No. 579608 disclosed a “Light Emitting Element for Forming Electrode and Method for Fabricating the Same”, wherein a metallic or metal alloy ohmic contact point is formed on a P-type gallium nitride semiconductor layer firstly, and a light-permeable oxide film is formed on them next. For another example, R.O.C. patent No. I240443 disclosed an “LED and Method for Fabricating the Same”, wherein a superlattice stress contact layer is formed on a P-type gallium nitride semiconductor layer firstly, and a transparent conductive layer is formed over them next.
How to achieve uniform current distribution under high current injection is always a focal topic in the field of large-area gallium nitride LED and solid-state illumination. The P-type current spreading layer 15 can indeed improve the current crowding in the P-type semiconductor layer 14 and lower the sheet resistance thereof. However, current crowding is also affected by the N-type semiconductor layer 12. The current crowding in the N-type region causes non-uniform carrier injection to the light emitting active region 13 and results in overheating in local area in the element, which reduces the internal quantum efficiency and decreases the light emitting efficiency.